Abstract The Ross Ice Shelf Polynya (RISP) is one of the primary production sites of the High Salinity Shelf Water (HSSW), the precursor of Antarctic Bottom Water (AABW) that drives the lower limb of global meridional overturning circulation. Utilizing a multi‐platform historical data set gathered over the western Ross Sea continental shelf, this study employs a box model to estimate heat and salt budgets in the western RISP and Ross Ice Shelf Cavity (RISC) system. Our analysis identifies three dominant controls on heat and salt budgets in the western RISC: cavity‐polynya exchange (EX), glacial basal melting (GM), and subglacial discharge (SD). The rate of SD is estimated at 87.0 Gt·yr−1, higher compared with previous studies. This study highlights the role of subglacial discharge in modulating water mass evolution in Antarctic marginal seas, a component that requires incorporation in future climate models.